One example of the related art will be described below with reference to FIG. 21.
The structure of this example will be described first.
FIG. 21 shows the configuration of a conventional liquid crystal display having optical detecting means. Each pixel consists of a liquid crystal element 201, a storage capacitor 202 and a pixel switch 203. One end of the storage capacitor 202 is connected to a constant voltage line 204, and the other end is connected to one end of the pixel switch 203. The other end of the pixel switch 203 is connected to a signal line 206, and its gate is connected to a gate line 205.
So far is the configuration of a usual liquid crystal display, but this example is further provided with an optical detecting element 211 consisting of an optical detection TFT (thin-film transistor) 207, an optical signal electric-carrier capacitor 208 and a scan switch 209. The gate of the optical detection TFT 207 here is connected to a constant voltage line 204, and one end of the optical detection TFT 207 is connected to a scan switch 209 and at the same time to the constant voltage line 204 via the optical signal electric-carrier capacitor 208. The other end of the scan switch 209 is connected to an optical signal output line 210, and its gate is connected to the gate line 205. The other end of the optical signal output line 210 is entered into an integrator composed of a feedback capacitor 213, a reset switch 214 and a differential amplifier 212.
Next will be described the operation of this example of the related art.
Its operation is the same as that of a usual liquid crystal display as far as a signal voltage entered via the signal line 206 is written into the storage capacitor 202 in the pixel scanned by the gate line 205 and an image is displayed by the liquid crystal element 201 by manifesting optical characteristics according to the signal voltage.
Hereupon in this example of the related art, the optical signal electric-carrier capacitor 208 is scanned at the same time by the scanning of the gate line 205, and the optical signal electric-carrier stored in the optical signal electric-carrier capacitor 208 is entered into the integrator via the optical signal output line 210. The integrator buffers the signal electric-carrier stored in the optical signal electric-carrier capacitor 208 and outputs it as a voltage Vout. By this operation, the conventional display device can not only visually display the display signals but also output the optical image coming incident on the display screen as electric signals (see Non-Patent document 1 for instance).
[Non-Patent document 1] 2003 SID Digest of Technical Papers, pp. 1494-1497